Wireless networks standardized by Third Generation Partnership Long Term Evolution (3GPP LTE) implement ARQ (Automatic Repeat Request) or hybrid-ARQ (HARQ), from which HARQ also includes forward error correction. HARQ is used in HSDPA and HSUPA, which provide high-speed data transmission for mobile phone networks such as UMTS, and in the IEEE 802.16-2005 standard for mobile broadband wireless access, also known as “mobile is WiMAX”. It is also used in EVDO and LTE wireless networks.
In systems of this type, terminals are required to send acknowledgment feedback to the network indicative of a result of decoding a transport block or codeword (ACK/NACK or ACK/NAK feedback). The ACK/NACK related to downlink transmissions is transmitted on the uplink. The feedback is used to trigger fast retransmissions. In LTE frequency-division duplex (FDD), as schematically depicted in FIG. 6, a terminal is required to transmit ARQ or hybrid-ARQ acknowledgment related to downlink subframe n in uplink subframe n+4. In FIG. 6, Tp denotes a propagation delay from the access node to the terminal; TTA denotes an offset separating the start of an uplink subframe relative to the start of a corresponding downlink subframe at the terminal; TUE is the processing time available to the terminal; and TeNB is the processing time available to the access node. This allows the terminal between 2 and 3 ms for decoding the transport block and preparing the uplink transmission that carries the ACK/NACK. The exact time depends on timing advance settings.
FIG. 7 illustrates a timing relationship between downlink data and uplink hybrid-ARQ acknowledgement for time-division duplex (TDD). In fact, the acknowledgement transmitted in uplink subframe 7 is bundled, and will be positively valued only if both of the downlink transmissions in subframe 0 and 3 are correctly decoded.
In LTE, as outlined in FIG. 8, a transport block comprises one or more code blocks. Each received code block needs to be correctly decoded, i.e. decoded without detected errors, in order for the transport block to be deemed correctly decoded. A cyclic redundancy check (CRC) value is inserted into each code block. The CRC makes it possible for the terminal to determine whether it has been correctly decoded or not.
It is expected that future radio access (“5G”) transport blocks will be structured in a similar way. For some services foreseen for 5G, however, the delay associated with ACK/NACK signaling as currently practiced in LTE will not be acceptable. More precisely, it is expected that the time allowed for the terminal to decode a transport block and transmit ACK/NACK feedback may be significantly reduced compared to LTE, possibly down to tens of microseconds. In some proposed deployments of 5G networks, terminals may be required to send ACK/NACK feedback to the network before any further downlink or uplink signaling can take place. This means that to the frame structure must leave some guard time to allow for decoding of the transport block. In a typical implementation, the time required to decode a code block once all its OFDM symbols have been received corresponds approximately to the duration of one OFDM symbol. Guard times of this order of magnitude would be wasteful of system resources. It is therefore desirable to expedite the terminal's transmission of ACK/NACK feedback.